The present invention disclosed herein relates to semiconductor devices and more particularly, to input/output devices including level shifters.
In a general system-on-chip (SOC), power is supplied into blocks by means of plural power source voltages. This is because internal blocks of an SOC are different from each other in operation voltage. As these blocks with different operation voltages are also different from each other in signal level, there can be problem related to signal transmissions. For instance, in a case that an operation voltage of a CPU is 1.2V and an operation voltage of an input/output device is 3.3V, the input/output device can regard even a high level signal of 1.2V, which is transferred to an external system through the input/output device from the CPU, as a low level signal, which causes a functional error therein. Further, owing to differences of operation voltages, there would be leakage currents toward the CPU from the input/output device. To solve those problems, an input/output device usually includes a level shifter or a level conversion circuit.
If the SOC is in a normal mode, the level shifter receives an internal power source, which is an operation voltage of the CPU, and an operation voltage of the input/output device and converts the input internal power source voltage into the same level as the operation voltage of the input/output device. Thus, the level shifter is helpful to preventing the leakage current and functional error due to different operation voltages between blocks in the SOC.
Otherwise, if the SOC is in an initial state of a voltage supply or in a specific state (e.g., a sleep mode), the CPU turns to an off-state to interrupt the power supply. But the input/output device maintains an output value of a fixed level for interface with the external system and supplies power in order to keep an on-state.
As the CPU is in the off-state while the input/output device is active, the level shifter is supplied with power of the input/output device, not supplied with the internal power. Accordingly, an input of the level shifter is floated and an output of the level shifter is set to a value. Even though the SOC is conditioned in the initial voltage supply state or the specific state (e.g., a sleep mode), the input/output device is required to have an output value with a fixed level because it has to be in an on-state for interfacing with the external system. But, as an input of the level shifter is being floated, its output is set to a value that is not the fixed level. Thus, even though a system requires an output value with a fixed level, i.e., although it requires a fixed output value of a low level, another fixed value of a high level would be output therefrom. As a result, the system cannot be assured of reliable operation in input/output processing.